Hot-dip galvanizing is applied to steel sheets to provide at corrosion prevention and the hot-dip galvanized steel sheets and hot-dip galvannealed steel sheet are widely used in building materials, household electric appliances, automobiles, etc. As one of the production methods, Sendzimir processing is a method comprising the processes of, in a continuous line in order: degreasing cleaning; heating a steel sheet in a non-oxidizing atmosphere; annealing it in a reducing atmosphere containing H2 and N2; cooling it to a temperature close to the plating bath temperature; dipping it in a molten zinc bath; and cooling it or cooling it after forming an Fe—Zn alloy layer by reheating. The Sendzimir processing method is widely used for the treatment of steel sheets.
As for the annealing before the plating, a fully reducing furnace method is employed sometimes, wherein annealing is applied in a reducing atmosphere containing H2 and N2 immediately after degreasing cleaning, without taking the process of heating a steel sheet in a non-oxidizing atmosphere. Further, employed also is the flux method comprising the processes of: degreasing and pickling a steel sheet; then applying a flux treatment using ammonium chloride or the like; dipping the sheet in a plating bath; and then cooling the sheet.
In a plating bath used in those processing methods, a small amount of Al is added to deoxidize the molten zinc. In the Sendzimir method, a zinc plating bath contains about 0.1% of Al in mass. It is known that, as the Al in the bath has an affinity for Fe stronger than Fe—Zn, when a steel is dipped in the plating bath, an Fe—Al alloy layer, namely an Al concentrated layer, is generated and the reaction of Fe—Zn is suppressed. Due to the existence of an Al concentrated layer, the Al content in a plated layer obtained becomes generally higher than the Al content in a plating bath.
Recently, demands for a high strength plated steel sheet excellent in workability are increasing in view of an improvement in durability and a weight reduction of a car body intended to improve the fuel efficiency of an automobile. Si is added to a steel as an economical strengthening method and, in particular, a high-ductility high-strength steel sheet sometimes contains not less than 1% of Si in mass. Further, a high-strength steel contains various kinds of alloys and has severe restrictions in its heat treatment method from the viewpoint of securing high-strength by microstructure control.
Again, from the viewpoint of a plating operation, if the Si content in a steel exceeds 0.3% in mass, in the case of a conventional Sendzimir method which uses a plating bath containing Al, plating wettability deteriorates markedly and non-plating defects are generated resulting in the deterioration of appearance. It is said that the above drawback is caused by the concentration of Si oxides on a steel sheet surface during the reducing annealing and the poor wettability between the Si oxides and molten zinc.
In case of a high-strength steel sheet, the added elements are abundant as explained above, and therefore the alloying heat treatment after plating is apt to be applied at a higher temperature and for a longer time than in the case of a mild steel. This is one of the obstacles to securing good material quality.
Further, from the viewpoint of an improvement in the durability of a structural member, fatigue resistance, in addition to corrosion resistance, is also important. That is, it is important to develop a high-strength steel sheet having good plating producibility, good fatigue resistance and good corrosion resistance simultaneously.
As a means of solving the problems, Japanese Unexamined Patent Publication Nos. H3-28359 and H3-64437 disclose a method of improving plating performances by applying a specific plating. However, this method has a problem that the method requires either the installation of a new plating apparatus in front of the annealing furnace in a hot-dip plating line or an additional preceding plating treatment in an electroplating line, and this increases the costs. Further, with regard to fatigue resistance and corrosion resistance, though it has recently been disclosed that the addition of Cu is effective, the compatibility with corrosion resistance is not described at all.
Further, Si scale defects generated at the hot-rolling process cause the deterioration of plating appearance at subsequent processes. The reduction of Si content in a steel is essential to suppress the Si scale defects, but, in the case of a retained austenite steel sheet or of a dual phase steel sheet which is a typical high ductility type high-strength steel sheet, Si is an additive element extremely effective in improving the balance between strength and ductility. To cope with this problem, a method of controlling the morphology of generated oxides by controlling the atmosphere of annealing or the like is disclosed. However, the method requires special equipment and thus entails a new equipment cost.
Yet further, when high-strength steel sheets are adopted for the purpose of achieving weight reduction by the reduction of the sheet thickness and the thinning of the steel sheets proceeds, more enhanced corrosion resistance may sometimes be required of even hot-dip galvanized steel sheets or hot-dip galvannealed steel sheets. For instance, an environment where rock salt is sprayed as a snow melting agent is a severe environment because it contains a comparatively large amount of Cl− ions. In the case where a plated layer exfoliates locally at the portions which are subjected to heavy working or the plated layer itself has insufficient corrosion resistance, a base material with excellent corrosion resistance and the formation of a plated layer with excellent corrosion resistance are required.
A steel sheet, which allows weight and thickness reduction and is prepared taking into consideration strengthening, the problems related to Si and improvement in corrosion resistance, has not been developed.
Yet further, while aiming at improving the producibility in plating a high-strength steel sheet, Japanese Unexamined Patent Publication No. H5-230608 discloses a hot-dip galvanized steel sheet having a Zn—Al—Mn—Fe system plated layer. However, though this invention particularly takes the producibility into consideration, it is not such an invention that takes the plating adhesiveness into consideration when a high-strength high-ductility material is subjected to a heavy working.
Furthermore, aiming at enhancing the collision energy absorbing capability, Japanese Unexamined Patent Publication No. H11-189839 discloses a steel sheet: having the main phase comprising ferrite and the average grain size of the main phase being not more than 10 μm; having the second phase comprising austenite 3 to 50% in volume or martensite 3 to 30% in volume and the average grain size of the second phase being not more than 5 μm; and containing bainite selectively. However, this invention does not take plating wettability into consideration and does not provide the corrosion resistance which allows thickness reduction accompanying increased strength.